Catalysts for dry reforming of methane

The mitigation and utilization of greenhouse gas have become the most significant challenges in the area of green energy research. One feasible solution is the catalytic reforming of methane with carbon dioxide (called dry reforming of methane, DRM) that converts the two main greenhouse gas (CO2 and CH4) into synthesis gas (H2 and CO, H2/CO ratio of 1:1), which is a resource for the manufacture of useful value-added products such as methanol, ammonia and synthetic hydrocarbon fuel production. DRM is an endothermic reaction that needs to be operated at high temperatures as to achieve considerable conversion. The main reaction of DRM is favorable at a temperature > 727 °C. However, if the reverse water gas-shift (RWGS) reaction occurred concurrently with DRM reaction, it may lead to a decrease of H2/CO ratio [1].In dry reforming reaction, the most utilized catalysts can be divided into two types: non-noble metal (typically Ni or Co based) and noble metals (typically Rh, Ru, Pd, Pt and Ir based). All of these metals responsible in C–H and C–C bonds cleavage, which is crucial step for the dry reforming. Noble metals were highly active for DRM with high resistance to carbon formation. Ni- and Co-based catalysts were studied because of their cost effectiveness. They showed activity comparable to that of noble metal-based catalysts, but they rapidly deactivated on account of either carbon formation or sintering. Thus, to date, both noble and non-noble metals have been used as active metals, and noble metals are utilized as a promoter in certain cases. Among these metals, Ni is the usual choice of active metal for commercialized steam reforming reactions at industrial scale [2, 3]. The role of the support is literally to provide high surface area (SiO2 and Al2O3) for the dispersion of the active metals. Furthermore, the stability of the support is very important in DRM because it operates at high temperatures. In addition to the conventional effect of the support, the support also plays an active role in the catalytic reaction. The support provides certain physicochemical properties such as basicity (CaO, La2O3 and MgO), oxygen storage capacity (CeO2, CeO2-ZrO2 and TiO2) and reducibility (CeO2 and ZrO2), which account for their intrinsic behavior in catalytic reactions, and also have implications for their resistance against carbon formation [4].The main issue that needs to be addressed for DRM is the deactivation of catalysts by sintering and carbon formation. Design of a viable catalyst that exhibits high catalytic activity and stability, as well as resistance against deactivation, could be accomplished by making appropriate choices of active metal, support, promoter, structure and methods for preparation and activation [5]. The development of highly active and stable catalysts with resistance against deactivation and economic feasibility is very important for successful industrialization of the DRM process. In general, active metals, supports, promoters, structure and methods for preparation and activation should be considered in designing an appropriate catalyst.It should be emphasized that the factors affecting syngas production not only depends on the nature of the active metals and supports, but also on the operation conditions such as reaction temperature, feed compositions, pretreatment temperature and flow rates of the reactants. Both reaction activity and product distribution are strongly affected by reaction conditions. It should be noted that reaction parameters are varied depend on the nature of the catalyst (active metal and supporting material). Therefore, optimizing of these reaction parameters are required to boost the reaction performance. This can be done by applying thermodynamics analysis and attempting on response surface methodology (RSM). The CO2/CH4 reactant ratio, reaction pressure and reaction temperature had a considerable influence on the equilibrium of the reactants conversion and solid carbon formation. Analysis of results of all these aspects demonstrated that optimum conditions for DRM process are as follows: Temperature range 643-1023 ⁰C, Pressure 1 atm, Feed ratio CO2/CH4 1:1 [2,6].

Keywords:

dry reforming of methane (DRM) , noble metals , non-noble metals , Ni catalyst , carbon formation